The present invention relates to a process for the depolymerization of polycaprolactam processing waste to form caprolactam.
The processing of polycaprolactam into intermediate articles such as fiber, chip, film, or molded articles results in polycaprolactam (hereinafter "nylon 6") processing waste, i.e., scrap nylon 6 polymeric and/or oligomeric materials. Examples of such scrap nylon 6 polymeric and/or oligomeric material are yarn waste, chip waste, or extruder slag. Examples of scrap nylon 6 oligomeric materials are the linear and cyclic oligomers of caprolactam. The nylon 6 intermediate articles are then incorporated or transformed into end use products such as fabrics, engineered plasics, carpets, and packaging.
The current worldwide production of polycaprolactam is enormous and this polycaprolactam is then processed into the intermediate articles. The scrap nylon 6 which results from this polycaprolactam processing into intermediate articles is sizeable.
In order to improve the yield in the processing of polycaprolactam, the scrap nylon 6 materials are depolymerized to caprolactam and the caprolactam is then reused. Recovery of caprolactam from polycaprolactam processing waste scrap, i.e. nylon 6 which is substantially free of non-nylon 6 materials, has been practiced for at least twenty years. In general, nylon 6 is depolymerized by heating at elevated temperatures, usually in the presence of a catalyst and/or steam. See U.S. Pat. Nos. 4,107,160; 5,233,037; 5,294,707; 5,359,062; 5,360,905; 5,468,900; German 4,421,239A1; Example 5 of European Patent Application 608,454; and Chem. Ing. Techn. 45, 1509 (1973). The caprolactam produced may be removed as a vapor stream as taught by AlliedSignal's U.S. Pat. No. 3,182,055. In most of the above processes a catalyst such as phosphoric acid is used to promote depolymerization of polycaprolactam. An extensive review of the field has been given by L. A. Dmitrieva et al, Fibre Chemistry, Vol. 17, No. 4, March 1986, pp. 229-241. U.S. Pat. No. 5,495,014 teaches the depolymerization of nylon 6 wherein the reaction is in the liquid phase at elevated temperatures in the presence of a heterogeneous catalyst and in organic solvent.
U.S. Pat. No. 3,939,153 to Fowler teaches a polycaprolactam depolymerization process wherein superheated steam and melted scrap nylon 6 are combined in a tubular elongated reactor. The reference teaches that the average temperature in the reactor is about 343.degree. C. to about 677.degree. C., the average residence time for the nylon melt in the reactor is from about one to about 40 minutes, and the average residence time for the superheated steam in the reactor is from about 0.01 to about ten seconds. The reference makes no mention of pressure in the reactor. The steam and nylon melt decomposition product then pass as a combined stream out of the reactor into a nylon column wherein the steam and nylon decomposition products pass overhead in vapor phase and the unconverted nylon 6 and byproduct oligomers are withdrawn from the nylon column and recycled back to the feed tank. The reference teaches that in a typical system, about 20% of the nylon 6 passing through the reactor is depolymerized to caprolactam and the remainder is recycled back to the feed tank.
The Fowler polycaprolactam depolymerization process which has only a 20% first pass yield is unacceptable in industry. Additionally, we have found that under the process conditions employed by Fowler, relatively large amounts of caprolactam cyclic dimer and ammonia are produced. Thus, a need exists in the industry for an improved process for the depolymerization of polycaprolactam processing waste.